Tuning of electrical resonators



DIZHH'UH HUUH y 9 1951 H. G. COOPER ET AL 2,561,727

TUNING OF ELECTRICAL RESONATORS Filed July 7, 1943 E as E u E 52 55 t 52ss 52 3| 3 5 HAROLD e. COOPER H UH HOWARD-L. SCHULTZ l abhor/wagPatented July 24, 17951 gutnvl l nvviu UNITED STATES PATENT OFFICETUNING OF ELECTRICAL RESONATORS Harold G. Cooper, Belmont, and Howard L.Schultz, Wellesley, Mass, assignors, by mesne assignments, to the UnitedStates of America as represented by the Secretary of the NavyApplication July 7, 1943, Serial No. 493,778

7 Claims. 1

This invention relate to apparatus for generating oscillations of veryhigh frequency, especiallyoscillations of frequency of the order of 2500mc./sec., 300 mc./sec. and higher. More particularly, the inventionconcerns arrangements for tuning resonator apparatus adapted foroperation in' the said frequency range with vacuum tubes of the triodetype, particularly vacuum tubes with close-spaced plane'elementsconstructed in a manner such as to bring the" grid connection outsidethe vacuum' tube to a ringshaped terminal surface.

For the production of stable oscillations in the above-mentionedfrequency range with triode vacuum tubes, resonator apparatus hascome'into use comprising three coaxial conductors, an out r cylinderconnected effectively to the cathode, an inner cylinder connectedeffectively to the anode, and an intermediate cylinder connected to thegrid. The outer and the inner cylinder electrically define a closedspace and the intermediate cylinder, partially enclosin the innercylinder in the neighborhood of the anode connection, acts as apartition in such space. The manner of operation of such resonatorapparatus will be more fully explained below. Tuning of such resonatorapparatus has heretofore been accomplished in various ways chiefly byintroducing retractable or otherwise adjustable masses of metal intoportions of the resonator having a relatively intense electric field. Ingenefi'al, such tuning arrangements have been limited in scope by thenecessity of avoidin breakdown discharges in the resonator and it hasbeen founddiflicult to obtain a sufiiciently wide range of vaigiation ofthe natural frequency of the device with any single adjustment.

It is an object of the present invention to provide improved tuningarrangements 'for resonators of the above type employed for thegeneration of electrical oscillations of very high frequency. 'It is afurther object of the present invention to provide a resonantcircuitstructure for ultra-high-frequency triode oscillators capable ofj being tuned over a relatively wide range bythe: variation of a singleadjustment. Other objects of this invention will be apparent fromreading of, this specification.

, An oscillator circuit structure embodying the present inventionisshown in the annexed drawing, Fig. 1 being a longitudinal cross-sectionand Fig. 2 a transverse cross-section in the plane of the grid terminal.Inorder that the manner in which the tuning apparatus'functions may beclear, the mode of operation of the circuit struc- ,2 ture and theassociated vacuum tube will be explained. 'I

In Fig. 1 the base'of a triode tube adapted for operation at theabove-mentioned "frequency is shown at 5, the outer part of the basehaving a metal case or shell. "Although, the circuit structure orresonatoris shown in cross section. the vacuum tube is forconveniencej'of illustration shown in side elevation in Fi l. r,

The elements of the=vaciium tube are provided with their effectivesurfaces located in closely spaced parallelpl'ane (not shown). vThe gridelectrode is continued outward from the grid plane towards the wall ofthe vacuum tube-to form a ring-shapedcontact surface on the outside ofthe tube intermediate the ends thereof, shown in the-drawing atG. Theglass 'walls'of the tube appear at and'il, being cylindrical'in shape,the upper cylinder 8 having a smaller diameter than the cylinder 1, "afeature which has earned for this type of'tube the appellation orlighthouse tube. The plate connection of the tube is brought out .to adisk 9 at the upper end of the cylinder 8, the disk 9 carrying also acapor stud H! which serves as the anode connection.

of the tube. The cathode is connectedby a suitable by-pass capacitanceto the metallic shell 5 which surrounds the base of the'tube, the directconnections to the cathode and its heater-being brought out to the basepins I l.

The resonator or tuner structure, likemost circuits for producing 'or'receiving oscillations at the frequency range in question, comprises amore or less developed form of cavity resonator having conductingsurfaces of considerable ex tent, on which the oscillatory currentsaredesigned to flow. The resonator i so connectedto the elements of thevacuum tube as to'prov'ide the necessary conditions for the generationof oscillations when the electrode voltages are provided in the usual.way.

The resonator structure comprises an outer tubular member l5 adapted tobe connected to the base shell 5, which in turn is by-passed to thecathode, an inner cylindrical member l6 adapted to be connected to theanode connection Ill, which member may be. hollow if desired, and acylinder or sleevegllj of; a diameter intermediate between theouterdiameter of the cylinder l6 and the inner diameter of the'tubular'member l5, usually less than the mean of those diameters, whichis adapted to be 'mounted upon the grid terminal ring 6 and to be heldin place thereon by one ormore screws l8, l8a threaded in the tubularmember l5 (see Fig. 2). With the assistance of spring action resultingfrom slots 5I cut in the lower end of the sleeve II, a slight annulargroove being cut on the inside of the slotted portion to allow thesleeve I1 to snap onto the terminal, the screws I8, I81; may provide anelectrical connection between the sleeve I1 and the tubular member I5,in which case the apparatus is operated as a grounded-grid oscillator,or they may be mounted on insulating bushings in the member I5 which areadapted to isolate the screws I8, I8a electrically from the member I5and thus to permit a grid bias or grid modulation to be applied to thegrid of the vacuum tube by an electrical connection through one or bothof the screws I8, I8a.

Apparently most important in the determination of the frequency ofoperation is the length of the sleeve I'l. Although the theoreticalexplanation of the operation of the apparatus in question is not as yetcertain, it is believed that the sleeve I1 so divides the space betweenthe cylinder I6 and the cylinder I5 as to provide a folded coaxialresonator one end of which is connected across the grid-plate space ofthe vacuum tube the other end of which is connected across thegrid-cathode space of the vacuum tube. The upper end of the sleeve I1 isfound to be at or very near to a voltage maximum of the resonator. Inorder to obtain the proper proportioning between the parts of theoscillating cavity, it is important to provide a small downwardextension I9 of the cylinder II extending from the annular grid terminal6 axially towards the base shell 5. For best results, especially forshort-pulse intermittent operation, great care should be exercised inthe proper design of the length of the extension I9. It appears to havesome dependence upon the inter-electrode capacitances of the vacuum tubein question. For the type (EL-464 a length of about .02 wave lengthappears to be preferable for conditions of current short-pulseoperation. For continuous operation the length of the overhang ordownward extension is not particularly important. For the type (EL-464tube and conditions of continuous operation, we prefer an overhang ofabout .05 wave length. In general, the overhang should be between 0 and0.1 wave length.

It is possible that the length of the overhang extension I9 of thecylinder II, dimension a in the drawing, may exercise some control onthe degree of feed-back coupling to the grid of the vacuum tube. It isto be noted, however, that great difficulties are to be encountered inthe attempted explanation of oscillatory circuits of this character interms of conventional circuit theory.

It has been observed that the electric field is particularly strong inthe neighborhood of the upper end of the sleeve I1. This region istherefore adapted for the introduction of a loading capacitance theadjustment of which may be expected to have a tuning effect upon thesystem, and if desired an auxiliary tuning arrangement might be providedhere in the form of a retractable machine screw or the like, but it ispreferable to omit such arrangements because the tuning adjustmentdescribed below provided in accordance with this invention is fullyeffective for such tuning as is usually desired and the provision ofadditional tuning means would only complicate the calibration of thedevice.

It is also observed that an intense magnetic field exists near the baseof the cylinder I! in the neighborhood of the plane of the grid terminal6. This region of the structure is therefore appropriate for magneticcoupling to an outside circuit and accordingly the loop 23 and thecoupling transmission line 24 are provided in this part of the structureas shown on the drawing. The coupling line 24 is of the coaxialconductor type having an inner conductor 25 and an outer conductor 26.

Electrostatic coupling may be provided instead by the use of a probeextending into the resonator cavity, and indeed this may be located atthe same position as the loop 23, such probe being connected to theinner conductor 25 instead of the loop 23. The penetration of the probeinto the cavity may be made adjustable. Although the electric field isnot particularly intense at this location, this coupling is of adesirable type because of the fact that the line 25, 26 will normallypresent a low impedance.

The circumferential position of the screws I8 and I8a has been known toexert an important influence upon the reliability and power obtainablein the operation of the device. A preferred location of these screws isshown in Fig. 2, which is a cross section of the apparatus of Fig. 1 inthe plane of the grid. The grid terminal appears at 6, the grid meshbeing shown diagrammatically at 55. The slotted portion of the cylinderI1 is shown fitted snugly about the rim of the grid terminal 6. Two ofthe slots are omitted in order not to interfere with the screws I8 andI8a. The points of the machine screws I8 and I8a are preferably engagedin slight conical depression in the cylinder I1, thus providing for firmcontact and good mechanical conditioning.

If only one screw is to be used, the screw I8a should be omitted and theremaining screw should be located at an angular or circumferentialdistance from the coupling loop 23 of the order of magnitude shown inFig. 2 (some variation being possible without appreciable change inresult). Although apparatus of the type described can successfully beoperated with only a single screw, such as the screw I8, we prefer theuse of two screws, as shown, because of superior performance of thedevice when so constructed. When two screws are used, the screw I8should be located approximately as above mentioned and the screw I8ashould be located at a circumferential distance of approximately degreesfrom the screw I8. The exact reason why the particular arrangement ofthe screw or screws connecting with the cylinder I'I should be importantis not known, but it is believed that some sort of an auxiliary circuithaving reactive effect, perhaps having some unexplained magneticcoupling with the oscillating field of the resonator, may be con:stituted when one or more screws are introduced and brought into contactwith the cylinder IT.

The length of the sleeve II, exclusive of the extension I9, as shown bythe dimension D in the drawings should be from about 0.25 to about 0.4wave length, a length of about 0.34 wave length being preferred forcontinuous operation with the type Gil-446 tube and a length of about0.37 wave length being preferred for pulsed operation with the type(EL-464 tube. The variation between continuous and pulsed" operationwill be pointed out more fully below.

The diameter of the cylindrical elements of the structure of the presentinvention does not have much effect on the frequency, but has someeffect on the stability and power of the oscillations. Preferably themean diameter of the cylinder I1 is approximately 1%; times the outerdiameter of the cylinder I6 or a little larger, and at the same timeabout /3 of the inner diameter of the cylinder I5, or a little less. Itis convenient to provide the outer diameter of the cylinder I6approximately equal to the outer diameter of the terminal disk 9 and toprovide the inner diameter of the cylinder I! at approximately the valueof the outer diameter of the terminal ring 6. In order that the cylinderI! may be properly aligned on the ring 6 it is provided with a thickenedportion 28 which is adapted to engage the edge of the terminal structureand with a slight annular groove (not clearly shown), immediately belowthe thickened portion, to provide a snap-on effect in cooperation withthe slots 5 I.

- The inner diameter of the tubular member I5 is preferablysubstantially larger than the outer diameter of the metal shell 5, whichis bypassed to the cathode of the vacuum tube. Accordingly, an endstructure 29 is provided for mounting the tubular member I5 on the baseshell 5. The structure 29 preferably carries a clamp ring 38 and aclamping screw 3I and the upper surface of the structure 29 ispreferably rounded, as at 32, since it is believed that discontinuitiesin this region, such as might be provided by sharp corners, ought to beavoided.

The upper end of the resonator is effectively closed off by means of thestructure 35 mounted on the sleeve I6. The structure 35 is adapted tofurnish a radio-frequency short circuit between the cylinder I6 and thecylinder I5. An actual short circuit is undesirable, both because it isdesired to adjust the position of the cylinder I6 and because thecylinder I6, being connected with the anode of the vacuum tube, is athigh potential with respect to ground, whereas it is desirable to keepthe outer parts of the structure, such as the cylinder I5, at groundpotential. The cylinder I6 is insulated from the cylinder I5 by means ofthe insulating end structure 31. The annular structure 35 effectivelyprovides a short circuit at radio frequencies in the range of thefrequencies of operation. The length of the annular cylindrical spacebetween the structure 35 and the cylinder I5 is made to be an electricalquarter-wave length, i. e., approximately a quarof the free-space wavelength for a frequency in the middle of the desired frequency range ofoperation, but slightly shorter on account of end effects. The Spaceenclosed between the structure 35 and the cylinder I6 will likewise havean axial length of an electrical quarter-wave length. Consequently, whenoscillations tend to appear across the gap 36, a high impedance willappear at the upper end of the structure 35 and consequently a verylowimpedance will appear at the lower end of the structure 36. Since theclearance between the cylindrical part of the structure 35 and thecylinder I5 is much smaller than the clearance between the cylindricalpart of the structure 35 and the cylinder I6, this low impedance effectat the gap 36 will take place over a range of frequencies in theneighborhood of that for which the axial length of the structure 35 isan electrical quarter-wave length.

The desired position of the structure 35 with respect to the upper endof the cylinder II has a relatively small effect, compared with thetuning effect described below, upon the frequency of the oscillation. Ifthe structure 35 is too close or too far away from the end of thecylinder I1, the effect is essentially to introduce a discontinuity inthe folded resonant transmission line, regarding the resonator as a.transmission line folded around the upper end of the cylinder I1, andsuch discontinuity may have to be kept within reasonable limits forproper operation of the resonator, it being understood that when theresonator operates so that this type of discontinuity is at a voltagemaximum, the discontinuity has relatively little effect. The desiredposition of the structure 35 is best determined experimentally. With thestructure 35 fixed on the cylinder I6 at a suitable position the smalltuning efiect produced by moving the structure 35 is added to thattuning effect described below operated by moving the cylinder I5, andthus serves to extend slightly the tuning range. We find that forapparatus constructed as shown in the drawing, the distance between thelower end of the structure 35 and the lower end of the cylinder I6, asindicated by the dimension 0 in the drawing, should for best results liebetween about 0.4 and 0.7 wave length. In apparatus designed forcontinuous operation with a vacuum tube of the type GL- 446, we preferto provide this length with a value of about 0.45 wave length, and forapparatus designed for recurrent short-pulse operation with a vacuumtube of the type GL-464, we prefer a length of about 0.61 wave length.

The relatively wide range tunin of the device shown in the drawingresults from the provision of an adjustable sliding anode connectionbetw'ee rf'the-l-ower end oftlijefc'ylindr l 6" and the anodestud"'IOI"'Whave found thatwhen the cylinder I6 is slid axially on theanode cap II) a remarkably strong effect on the resonant frequency ofthe device develops, through which it is possible to vary the saidresonant frequency over a considerable range. With anode studs of normalsize, such as about one-quarter of an inch in height, variations in wavelengths over a range of 12 mm. in the neighborhood of 10 cm. wavelengths have been found possible and it is believed that even greatervariation could be achieved with longer anode studs or with other minorvariations in the size and shape of the constituent .parts. In thistuning effect only about 2 mm. wave length change is contributed by themovement of the structure 35. The major tuning effect is believed toresult from the unusual configuration of the electric field in theneighborhood of the gap between the lower end of the cylindricalstructure I6 and the annular plug 38 on one hand and the annularterminal 9 and the stud II] on the other hand. Over a great deal of thetuning range the change in wave length is almost a linear function ofthe axial position of the cylinder I6.

In order that the tuning effect may be sufliciently strong to providewide range tuning, the cylinder I6 should be considerably greater indiameter than the anode stud Ill. An annular plug 38 may conveniently beprovided to engage the cylindrical surface of the anode cap II]. Inorder to provide a strong tuning effect, the annular gap or channelbetween the lower face of the plug 38 and the terminal surface 9 shouldhave a considerable radial depth. On the other hand, the diameter of theanode cap III should not be so greatly reduced as to impede heattransfer between the cap I0 and the plug 38, since it is desired toconduct heat away from the anode through the cap I0, the plug 38 and thecylinder I5.

An important practical measure in the construction of the tuningmechanism is the reduction of the thickness of the cylinder I6 as atoms?shown at 52 and the provision of the slots 53, which also cut the plug38 into segments. This t pe of construction of the lower end of thecylinder I6 provides a secure spring-pressed electrical contact togetherwith sufficient mechanical flexibility to avoid breaking of the vacuumtube as a result of repeated manipulations of the cylinder I6 forpurposes of tuning.

, The tuning arrangement shown in the drawing for tuning the resonatorof a triode oscillator device is believed to constitute a novel tuningadjustment for cavity type resonators, and especially for coaxialconductor type resonators. In general, the principle of this arrangementprovides a useful way of modifying the electrical properties of acoaxial conductor transmission line.

Although the tuning effect of axially moving the cylinder l6 withrespect to the anode stud I is very considerable, this adjustmentexercises relatively little influence upon the efiiciency or stabilityof the oscillating circuit, so that this tuning adjustment is especiallyuseful in practice. Unlike certain other types of tunin adjustments, thetuning adjustment resulting from the axial movability of the cylinder [6does not vary the voltage breakdown characteristics of the device. anddoes not reduce the clearances in the direction of the electric vectorin the course of the adjustment.

In order that the above-described unusual tuning properties may beconveniently availed of, the cylinder I6 is threaded at its upper endand engaged in bushing 4| which is suitably embedded in the insulatedterminating structure 31. The position of the cylinder l6 may beadjusted by turning a knob 42 mounted on the end of the cylinder I6 andpreferably made of insulating material. The knob 42 is provided with anaxial opening 43 through which a connection, preferably insulated, maybe introduced for electrical connections to the anode of the vacuumtube. If the cylinder I6 is made hollow, as shown in the drawing, meansfor cooling the anode might be introduced therein, if desired. Ingeneral the cylinder It will be made of copper, which has a suflicientheat conductivity to make special cooling means unnecessary. Especiallyfor the type (EL-464 tube, however, the connection at the lower end ofthe cylinder [6 to the anode stud l0 should be made in such a manner asto provide good heat conducting connections as well as good electricalconnections. The cylinders l5, l5, and I! and their associatedconducting structures are preferably made of copper or brass, and may besilver-plated on the surfaces carrying radio-frequency currents in orderto decrease losses. For good heat conductivity the central hole usuallyprovided in the cylinder IS in order to save weight should not be toogreat in diameter, nor should the reduction of the tubular wall shown at52 be carried any farther than necessary for mechanical reasons.

A knurled nut 44 of insulating material is provided near the upper endof the conductor l6 for maintaining any desired axial adjustment of thecylinder l6. Locking pairs of nuts 46, 41 and 48, 49 are also providedon the cylinder l6 for limiting the extent of the axial movement of thecylinder I6 so that it cannot be pushed downwardly so far as to exertundue pressure upon the disk 9 and the structure of the vacuum tube, andso that it cannot normally be retracted upward far enough to'losecontact with the anode stud l0.

A method which might be used for auxiliary variation of the operatingfrequency of the structure illustrated in the drawing consists inproviding solid dielectric material in part of the space between thecylinder I1 and the cylinder; l6. Thus a transverse annular mass ofpolystyrene located in said space, preferably near its upper end, willvary the operating frequency on account of its presence by virtue of thefact that such dielectric material changes the electrical length of thecoaxial line formed by the cylinders l6 and IT. The effect of thedielectric mass, moreover, varies with its axial position, probablybecause the intensity of oscillating electric field varies axially inthe space between the cylinders l6 and II. It is therefore possible toobtain some variation of the operating frequency by axially moving inthe said location an annular piece of dielectric material, such aspolystyrene or the like. Because of the greater mechanical inconvenienceof such an arrangement this method of varying the frequency is believedto have a limited applicability in practice but it may be useful as afactory adjust:- ment for setting the mid-tuning range frequency of thedevice at a desired value for a device intended to be tuned by means ofa sliding anode connection.

It is to be understood that the principles of the sliding anodeconnection tuning arrange-' ment may be applied in a device of the typeshown in the drawing by means of a sliding joint or other variableannular groove, a gap or channel constituted some other way than by aslid-.- ing anode connection. Thus the sliding joint might be providedbetween two portions of the cylinder It in the neighborhood of the anodestud In, an arrangement which might reduce the mechanical strain on thestructure of the vacuum tube. It is believed to be desirable to providethe variable discontinuity in the resonator at positions whereconsiderable current flows, although it is not necessary to provide itexactly at a location of current maximum, since it is likely that thetuning effect is due in a large measure to the current flowing aroundthe groove or gap. It will be noted that the groove or channel isoriented transversely to the direction of current fiow.

With oscillator apparatus such as that herein described stableoscillations tunable over a considerable range..ma .beiobtained byproviding the usual anode voltage to the circuit and providingthecathode with the desired heating potential. The "cathode terminal,which is brought out of the vacuum tube through one of the pins H may beconnected directly to ground or it may be provided with a suitablebiasing voltage. The oscillator may conveniently be modulated either byvarying the plate voltage or by varying the grid voltage, the gridvoltage being readily variable, as previously indicated, by providingthe screw l8 with an insulating mount and connecting the desired gridvoltage to the screw I8. Cathode modulation may also be accomplished byintroducing a modulating voltage between the cathode and ground. Theoscillator may also be operated to produce high intensity pulses ofshort duration in response to transient voltages impressed upon thecircuit in any of the above modulating methods. It is to be noted,however, that for a given circuit ar rangement and a given vacuum tubethe frequency of continuous operation differs from that of theoscillations occurring in high-intensity short-pulse operation, thelatter frequency generally being lower. In consequence it is desirableto take this feature into account in designing the apparatus of thisinvention in order that this manner of behavior of the circuit may becompensated for by the design of the resonating structure.

In the above-mentioned preferred values for the length of the sleeve I1and other dimensions of the apparatus of the present invention, thosegiven for the type (EL-446, which tube is practicularly suitable forcontinuous operation, having somewhat less power output than the GL-464and being thereby more suitable for use as a local oscillator in asuperheterodyne receiver, were the dimensions referred to the wavelength desired for continuous wave operation. The preferred dimensions,given for use in connection with the type (EL-464 tube, however, werereferred to the wave length obtained when operation is confined to onemicrosecond pulse approximately 500 microseconds apart from each other.This factor accounts for the major part of the lengthening of the sleevel1 and perhaps also for the displacement of the structure 35. Thereactive loading effect of the tube on the resonator apparently isdifferent for the two types of operation. The wave length dimensionsgiven above refer to fractions of the so-called free space wave lengthfor the frequency in question.

The displacement of the natural frequency of the apparatus resultingfrom operation in which the oscillations are produced in the form of onemicrosecond pulses at a recurrent rate of 2000 per second isapproximately of the order of about "20 per cent toward lower frequencyand reater wave length over the frequency obtained with the sameresonator structure under continuous oscillation. By introducing thisproportionality factor, ;the preferred dimensions for the apparatusof-the present invention for continuous oscillation in association witha type (EL-464 tube and for intermittent oscillation in association atype GIL-446 tube may readily be estimated.

What we desire to claim and obtain by Letters Patent is:

1. An oscillator for high frequencies including a three-element vacuumtube, a tunable circuit structure which includes a first cylindricalconductor slidably connected to the anode of said vacuum tube, a secondcylindrical conductor substantially coaxial with and partially enclosingsaid first cylindrical conductor connected effectively to the cathode ofsaid vacuum tube, a third cylindrical conductor substantially coaxialwith the aforesaid conductors and located therebetween, said thirdconductor being connected near one end to the grid of said vacuum tubeand extorimeans connected to said first cylindrical conductor foradjusting the axial position of said first cylindrical conductorrelative to said second cylindrical conductor and relative also to saidanode, whereby the configuration of an oscillating electrical field inthe neighborhood of said 10 anode is changed, thereby varying thefrequency of operation.

2. An oscillator for high frequencies including a three-element vacuumtube, a tunable circuit structure which includes first and secondcoaxial cylindrical conductors connected respectively to the anode andcathode of said vacuum tube, a third coaxial cylindrical conductorintermediate said first and second cylindrical conductors of a lengthbetween about 0.25 and about 0.4 wave length at a desired frequency ofoperation and connected at a small distance, not greater than 0.1 wavelength at said frequencies from one end to the grid of said vacuum tube,also having at its other end a substantially open-circuited.terurination, and means connected to said first cylindrical conductorfor adjusting the axial position thereof said adjusting means beingoperable to provide an annular groove of variable axial dimension in theimmediate neighborhood of said anode thereby to cause tuning of saidcircuit structure. 1

3. An oscillator for high frequencies including a three-element vacuumtube, a tunable circuit structure which includes a first cylindricalconductor connected by an axially slidable joint to the anode of saidvacuum tube, said joint being adapted to define a variable width annulargroove, a second cylindrical conductor substantially coaxial with andpartially enclosing said first cylindrical conductor, said secondconductor being connected eifectively to the cathode of said vaccuumtube, a third cylindrical conductor substantially coaxial with saidfirst and second conductors and located therebetween, said thirdconductor being connected near one end to the grid of said vacuum tube,two substantially radial metallic rod-like connections to said thirdconductor located near the grid of said vacuum tube at a circumferentialseparation of approximately from each other, said end of said thirdconductor farthest from said grid extending over, but spaced from, aportion of said first cylindrical conductor and extending to asubstantially opencircuited termination, said first and secondconductors being effectively joined together for the frequency ofoperation by a conducting structure located between said conductors at apoint'a substantial distance beyond the said open-circuited end of saidadditional conductor, means connected to said first conductor foraxially adjusting the position of said first cylindrical conductorrelative to said second conductor and relative also to said anode,whereby said axially slidable connection may be operated and thefrequency of operation thereby varied.

4. An oscillator for high frequencies including a multi-element vacuumtube, a, tunable circuit structure which includes substantially coaxialcylindrical conductors connected respectively to the anode and cathodeof said vacuum tube, a third coaxial cylindrical conductor intermediatesaid first-named cylindrical conductors connected near one end to thegrid of said vacuum tube and having at its other end a substantiallyopencircuited termination, and means for tuning said structurecomprising an axially slidable connection between one of saidcylindrical conductors and said anode for varying the configuration ofthe oscillator field in the neighborhood of said anode connection andmeans connected to that one of said first-named cylindrical conductorswhich is connected to said anode for adjusting the axial positionthereof.

5. A generator of high-frequency electrical oscillations comprising, amulti-element vacuum tube of the lighthouse type having external ringanode, cathode, and grid connections, first, second, and thirdconducting surfaces of revolution respectively connected to said anode,cathode and grid, said first conducting surface being slid ablyconnected to said anode connection to provide an annular channel ofvariable axial dimension in the immediate neighborhood of said anodeconnection, and means connected to said first conducting surface foradjusting the axial position of said first conducting surface relativeto said anode connection, thereby controlling said variable axialdimension.

6. Apparatus for tuning an oscillator of. the type which includes amulti-element vacuum tube and first, second and third coaxially disposedconducting cylinders respectively connected to the anode, cathode, andgrid of said tube, said apparatus comprising a conducting annular pluginserted in one end 'of said first cylinder and contacting said anode,and means connected to said first cylinder for adjusting the axialposition of said first cylinder relative to said anode,

thereby providing an annular cavity of adjustable axial dimensionssurrounding said anode.

7. High frequency apparatus including a multielement vacuum tube, first,second and third coaxial cylindrical conductors connected respectivelyto the anode, gridand cathode of-said vacuum tube, said first conductingcylinder being slidably connected to said anode to provide an annularcavity of variable axial dimension around said anode, and meansconnected to said first cylinder for adjusting the axial positionthereof relative to said anode thereby controlling said variable axialdimension and the frequency of said apparatus.

HAROLD G. COOPER.

HOWARD L. SCHULTZ.

REFERENCES 'crmn The followingreferences are of record in the file ofthis patent:

" UNITED STATES PATENTS Gurewitsch June 22, 1948 Certificate ofCorrection Patent No. 2,561,721 July 24, 1951 G. COOPER ET AL. I It. ishereby certified. that error appears in the printed Specification of theabove numbered potent requiring correction us'followsz Column 1, line 4,for "800 mc./soc. read 8000 vim/um; column 8, line 83, before gap strikeout 0,; column 9,1mo 65, for benig rend being; column 10,1ine 33, forvaoouum road vacuum; and that the said Letters Pubout should be read ascorrected above, so that the some may conform to tho-record of the cagein tho Patent Oflice.

Signed and sealed this12tb day of August, A. D. 1952.--

moms F. MURPHY,

Assistant aamminibpr of Pmm.

